The invention relates generally to a link chain for friction pulley transmissions, and more particularly to an endless, flat-link, articulated chain for continuously variable transmissions of the movable cone type.
Continuously variable transmissions (CVTs), also known as infinitely variable or step-less transmissions, have long been used in marine, industrial, and automotive applications. In a CVT, regardless of the application, the ratio between the rotational speed of a drive shaft and the rotational speed of a driven shaft may be continuously varied between two predetermined limits, rather than in discrete steps.
In a typical CVT, an endless chain frictionally couples a pair of pulleys, otherwise known as sheaves. One pulley is connected to the drive shaft, and the second pulley is connected to the driven shaft. Links connected by pins comprise the chain. Each pulley is comprised of two opposed cones, also known as disks, which are moveable axially towards or away from each other on both the drive and driven shafts. Generally, one cone is fixed to each shaft and the second cone moves axially on the same shaft relative to the fixed cone. Each chain pin may comprise a pair of rockers that pivot on each other. The two ends of the pin are frictionally coupled to the opposing surfaces of the cones, which load the pins. Axial motion of the moveable cone in a drive pulley or a driven pulley or both places the chain at varying radial positions with respect to the axis of the pulley. This motion varies the transmission speed ratio between a drive shaft and a driven shaft.
For aerospace electrical power generation, a generator must typically be driven at a constant speed to enable the generator to produce constant-frequency electrical power, usually at 400 Hz. A CVT may be used to drive a generator from the output shaft of a gearbox driven by a gas turbine engine of an aircraft. The CVT typically converts the variable engine rotational speed to the constant speed required to drive the generator. As in any aerospace application, one of the challenges lies in maximizing the strength-to-weight ratio of the chain. This helps to maximize the power-to-weight ratio of the CVT.
That is, the transmission chain is required to handle the power demands of the CVT, yet be compact and lightweight. Many prior art chains used endless articulated patterns of flat links, with the links connected by pins. These chains carried torque between the pulleys by means of frictional forces on the ends of the pins; the links transmitted the tension, or force. The torque-carrying capacity of a chain multiplied by the pulley speed equals the power transmitted. However, the link pattern that optimizes the strength-to-weight ratio of the chain is far from obvious.
Many marine and industrial transmission chains are available. These chains are made from standard materials, are relatively low in strength-to-weight ratio, and are not designed for aerospace reliability. In aerospace applications, manufacturers are willing to use higher-quality materials and optimum designs to achieve greater reliability and higher strength-to-weight ratio.
In some prior art chains, stronger, or robust, links are used near the outer edges of the chain. As used throughout herein, the term xe2x80x9crobustxe2x80x9d refers to a chain link that has the characteristic of being relatively stronger. Robust links may be made in one or more of the following three ways: they may be made of material of greater tensile strength, they may be formed by mating at least two thin links, and/or they may be made of thicker material.
The pattern of robust links repeats every two or three pins in some designs. In some prior art chains, however, certain links see excessive loads because they are segregated from other links. That is, these segregated links do not have sufficient adjacent links or robust links to carry the tension. This lack of support causes the segregated links to carry more than their share of the chain tension, causing higher stress, or tension per unit area, in the segregated link. In addition, certain pins experience higher bending, or twisting forces (hereinafter xe2x80x9cmomentsxe2x80x9d) leading to higher stresses and premature failure. This will be discussed in more detail hereinbelow.
Examples of prior art chains in which certain links experience excessive loads are found in U.S. Pat. No. 4,547,182 and 4,927,404, both to Rattunde. The chains of these patents attempt to balance the loading of a chain pin to prevent bending and twisting it. This was tried by placing robust links mostly at the edges, or mostly near the center. However, the tension distribution among the links is unequal when strong, or robust, links are used only at the outer or inner edges of the chain.
Accordingly, it is an object of this invention to provide a transmission link chain with improved strength-to-weight ratio.
Another object is to provide a link chain that balances stress and moments on the inner and outer links as well as on the pins.
Still another object is for fewer links to carry the tension between any set of pins, but for the chain, when used in a CVT, to have at least 10% higher torque-carrying capacity (which requires 10% higher tension capacity) than a prior art chain.
Another object is to provide a link chain with sufficient reliability for an aerospace CVT.
A major step in the invention is the recognition that placing robust links throughout the chain in an evenly distributed pattern produces a more equally loaded chain.
According to the invention, an endless loop chain having a central axis comprises a plurality of pins; the chain also comprises a plurality of links, each link further comprising it two faces parallel to the center axis, each link fashioned with two holes perpendicular to the faces, wherein the links are pivotally connected in stacks and in overlapping series by the pins fitted though the holes, and wherein robust and thin links are arranged in the direction perpendicular to the faces, such that robust links are distributed evenly in the direction perpendicular to the axis of the chain. In the present invention, the term xe2x80x9crobustxe2x80x9d refers to links that may be made in one or more of the following three ways: they may be made of material of greater tensile strength than xe2x80x9cthinxe2x80x9d links, they may be formed by mating at least two thin links, and/or they may be single links made of thicker material than thin links. According to one aspect of the invention, a pattern that balances link stresses and pin moments as nearly equally as possible uses thin links and robust links that are staggered in a diagonal pattern, stepping from pin to pin, across the chain as follows:
1,1,2,1,2,1,2,1,2,1,2,1,2,1,2,1,1
where xe2x80x9c1xe2x80x9d refers to a thin link and xe2x80x9c2xe2x80x9d refers to a robust link.
According to a second aspect of the invention, a pattern that balances link stresses and pin moments as nearly equally as possible uses a chain that is sixteen links wide, such that, along a line generally perpendicular to the center axis that zigzags from a first to a second adjacent pin and back, the links are stacked as follows:
1, 2, 21, 1, 2, 1, 1, 1, 1, 2, 1, 1, 2, 2, 1
where xe2x80x9c1xe2x80x9d refers to a thin link and xe2x80x9c2xe2x80x9d refers to a robust link.
The invention has utility because it increases the tension capability of the CVT chain while reducing its weight, thus increasing the chain""s strength-to-weight ratio.
The above and other objects, features, and advantages of this invention will become apparent when the following description is read in conjunction with the accompanying drawings.